It may be desirable to attach an audio system to a vehicle exhaust. This audio system may be directed at two main goals. A first is so-called engine harmonics addition (ASD). In other words, the speaker may generate noise so as to change or amplify the “revving” and/or running sounds of an engine. This can be useful for amplifying quiet engines, enabling them to be heard, and for improving the sound of an engine to provide, for example, a desirable v8 roar. A second is so-called engine harmonics cancellation, which may also be termed active noise cancellation (ANC). In this latter case, the audio system generates anti-sound, thereby reducing the noise associated the revving and running of the engine.
However, the inside of an exhaust experiences high temperatures during running of the vehicle. In diesel engines, the front of speaker may experience temperatures at high as 140° C., while in petrol engines this may be as high as 160° C. These elevated temperatures can be maintained for long periods, for example, during long journeys. In addition, speakers themselves generate heat. Exhaust interiors also undergo repeated heating and cooling, as vehicles are used and then parked, used and then parked repeatedly over their lifetime. In cold climates, cool temperatures may be well below freezing for long periods in winter months.
Furthermore, the interior of a vehicle exhaust system is subjected to a cocktail of assorted gaseous byproducts and particulates, including incompletely combusted hydrocarbons, carbon monoxide, carbon dioxide, nitrogen oxides and sulfur compounds. These are often incompatible with and/or harmful to audio system components.
These extreme conditions limit the use of audio systems to alter car exhaust sounds. In particular, the current designs for loud speakers are unable to withstand the conditions within the exhaust system. As a consequence, audio systems for vehicle exhausts are located outside the exhaust interior, for example attached to the outside of the exhaust pipe.
US2013/0202148 describes a muffler for an exhaust system. It is mounted in a housing on the exterior of the exhaust system, and the sound is allegedly transmitted and radiated into the exhaust stream via a connecting pipe.
A similar arrangement is described in US2014/0328493.